Thermal conductivity of MgB₂ in the superconducting state

We present thermal conductivity measurements on very pure and dense bulk samples, as indicated by residual resistivity values as low as 0.5 mW cm and thermal conductivity values higher than 200 W/mK. In the normal state we found that the Wiedemann Franz law, in its generalized form, works well suggesting that phonons do not contribute to the heat transport. The thermal conductivity in the superconducting state has been analysed by using a two-gap model. Thank to the large gap anisotropy we were able to evaluate quantitatively intraband scattering relaxation times of $\pi $ and $\sigma $ bands, which depend on the disorder in different way; namely, as the disorder increases, it reduces more effectively the relaxation times of $\pi $ than of $\sigma $ bands, as suggested by a recent calculation [1].